The search for hybrids between Myriophyllum sibiricum and M. spicatum (Haloragaceae) in North Eurasia allowed to clarify their morphological differences and distribution

Myriophyllum sibiricum and M. spicatum are sympatric in large part of their vast ranges in Eurasia and North America and have different habitat preferences only in East Europe and Siberia. Such absence of both geographic and ecological isolation should trigger extensive hybridization between these species that is indeed observed in North America. The hybrid could be reliably documented only with genetic markers, and in Eurasia this was done only in China. Using genetic markers (ITS2 region of nuclear ribosomal DNA and trnL-trnF intergenic spacer of cpDNA) we documented hybrids between M. sibiricum and M. spicatum on the North Asian Pacific islands with dynamic geological history and climate (Commander and the southern Kuril islands). The hybrid also originated in East Europe (Ivanovo Region) as a result of ecological isolation break due to rapid artificial habitat change. Using genetically verified material throughout temperate Eurasia we clarified morphological characters discriminating M. sibiricum and M. spicatum and, basing on them, refined the species distribution. For the first time we provide a full morphological characteristic of the hybrid between M. sibiricum and M. spicatum and suggest a name for it, M. × hybridum.